Support one continuous net_buf for TCP TX case

[MaochenWang1]

Is your enhancement proposal related to a problem? Please describe.
Currently in function tcp_out_ext, for TCP TX, it will use two separate net_buf, one contains ip and tcp header, another contains data payload.
It has two side effects:
(1) for running TCP throughput case, the num of net_buf should be twice of net_pkt, and consume more memory.
(2) non-continuous buffer will decrease the memcpy efficiency when copy to lower interface.

Describe the solution you'd like
Use single net_buf for TCP TX data when NET_BUF_DATA_SIZE is large enough.

[jukkar]

If memory is an issue, it is actually better to set CONFIG_NET_BUF_DATA_SIZE to smaller value so that buffers are utilized better if the amount of data to be sent is small. Other option is to use CONFIG_NET_BUF_VARIABLE_DATA_SIZE which uses malloc to allocate just enough memory to the data to be sent.